Why in Java 'final String' initialized as String.toString() is not considered as constant [duplicate]

Question

I wrote the following code in Java which runs fine :

public class test {
    public static void main(String[] args) {
        final String s1 = "s1" ;
        final String s2 = "s2" ;

        String s = "s1" ;
        switch(s) {
            case s1 : System.out.println("s1") ;
                break ;
            case s2 : System.out.println("s2") ;
                break ;
        }
    }
}

But when I write the following code :

public class test {
    public static void main(String[] args) {
        final String s1 = "s1".toString() ;
        final String s2 = "s2".toString() ;

        String s = "s1" ;
        switch(s) {
            case s1 : System.out.println("s1") ;
                break ;
            case s2 : System.out.println("s2") ;
                break ;
        }
    }
}

I get the following error :

test.java:8: error: constant string expression required
                    case s1 : System.out.println("s1") ;
                         ^
test.java:10: error: constant string expression required
                    case s2 : System.out.println("s2") ;

I am looking for an explanation for this as I could not understand why is the second code giving me this error.

Answer 1

Because "s1".toString() is not a compile-time constant expression. Only compile-time constants (or enum constant names) can be used as labels in a switch statement. See the Java Language Specification, Section 15.28 for the rules for what constitutes a constant expression. (And see JLS §14.11 for the rules for the switch statement.)